As is well known, the Emergency Alert System (EAS) is designed to allow for the rapid and widespread dissemination of information relating to a national or local emergency to the general public. EAS messages are transmitted for either national, state or local emergencies or other events. Examples of these emergencies or events include: severe weather watch/storm warning, flash floods, earthquakes/tsunami, and war or other “man made” emergencies.
The EAS was designed in part by the Federal Communications Commission (FCC) in cooperation with the National Weather Service (NWS) and the Federal Emergency Management Agency (FEMA), in order to support the roles of each organization. The FCC provides information to broadcasters, cable system operators, and other participants in the EAS regarding the technical and operational requirements of the EAS. Additionally, the FCC ensures that state and local EAS plans conform to FCC rules and regulations. The NWS provides emergency weather information to alert the public about dangerous or potentially conditions weather conditions or other natural events. FEMA provides direction for state and local emergency planning officials to plan and implement their roles in the EAS.
Since Dec. 31, 1998, cable systems that have 10,000 or more subscribers are part of the EAS. These cable systems have the capability to transmit emergency messages on all of their video channels.
Alerts sent via the EAS may arrive in the form of text, audio and/or video content. Depending on the message type, the subscriber's television or set-top box or terminal will display the message in the appropriate format and according to the prescribed method. State and Local area emergency messages may be transmitted by using EAS Header and End of Message Codes. In television environments, the FCC recommends that the codes be preceded by an announcement that informs listeners that an EAS transmission will occur.
Recently, network operators have begun to offer switched digital video (SDV) services over cable and other broadcast networks. Switched digital video (SDV) refers to an arrangement in which broadcast channels are only switched onto the network when they are requested by one or more subscribers, thereby allowing system operators to save bandwidth over their distribution network. In conventional cable or satellite broadcast systems, every broadcast channel is always available to all authorized subscribers. In contrast, a switched digital video channel is only available when requested by one or more authorized subscribers. Also, unlike video on-demand, which switches a singlecast interactive program to a user, switched digital video switches broadcast streams, making each stream available to one or more subscribers who simply join the broadcast stream just as they would with normal broadcast services. That is, once a switched service is streamed to a subscriber, subsequent subscribers associated with the same service group as the first subscriber can tune to the same broadcast stream. The switched digital video will often share the same resource managers and underlying resources with other on demand services.
One way to support switched digital video is to utilize the session manager to manage SDV and other sessions. For each channel change, the subscriber will set up a broadcast session with the session manager, which will determine if the requested channel is already being sent to the corresponding service group that the subscriber belongs to. The subscriber will be assigned to join the existing SDV session if the requested channel is available at the service group or assigned to a new SDV session if the requested channel is not available at the service group. The session manager will negotiate with the edge devices to allocate resources required for the session. The edge device (e.g., a digital modulator such as a QAM modulator) needs to dynamically retrieve the MPEG single program transport stream that carries the requested SDV program (likely via IP multicast) and generate the MPEG multiple program transport stream. As part of the session setup response message, the video tuning parameters such as frequency and MPEG program number are sent back to the subscriber to access the requested SDV channel.
Communication between the session manager and the subscriber is performed using an SDV Channel Change Message (CCM) protocol, which can be implemented as a proprietary protocol or using an open standard. Among other things, these protocols pass channel change message or request from the subscriber to the session manager. The session manager responds by sending a message that includes the necessary tuning information to the subscriber.
When an EAS message is sent to a set of subscribers, the subscribers are directed to tune to a specific frequency and program number to view the EAS message. As a result each subscriber sends a channel change message to the session manager. This may cause such a large number of subscribers to send channel change messages at the same time that the bandwidth of the network will not be able to support all the messages. A small number of the channel change messages will get through, but the rest likely will be lost. If the network includes a retry mechanism, the failed messages will be resent after a short random backoff period. This will cause still more network congestion due to the additional channel change messages being generated.
The occurrence of an EAS condition or event may give rise to other problems in addition to the generation of an excessively large number of channel change messages. For example, it may be difficult for the subscribers to re-tune to their previous SDV channels after the EAS condition has ended. In part this problem arises because network congestion may prevent the subscriber from communicating with the session manager, which will be unable to process the channel change requests to re-tune to the previous SDV channels.